Zr-TiO2 nanotubes are synthesized via a new surfactant-free sol-gel route in supercritical CO2. The morphology of he Zr-TiO2 nanotubes can be tailored by changing either the concentration of the starting materials or the acid-to-metal-alkoxide ratio. This synthesis procedure is simple and scaleable, using mild reaction conditions and a green solvent, and provides a high yield and high-quality nanotubes (see figure).